1. Field of the Invention
The present invention generally relates to common amplification television transmitters that amplify both aural and visual signals simultaneously, and more particularly, to a system and method for reducing residual modulation of the aural carrier typically caused in common amplification television transmitters.
2. Background Discussion
Aural carrier distortion is a systems problem which involves all of the elements of a television communication system, including the receiver as well as the transmitter. This unwanted distortion is caused by video components of the television signal modulating the visual carrier and because of the specific frequency bandwidth, those same video components affecting the aural signal in a different phase domain. This unwanted aural carrier modulation occurs in both the phase and amplitude domains and is related to the continuous modulation of the total signal envelope by the video signal components.
Specifically, the component of the video signal relating to the line scan frequency, which is found at 15.734 KhZ and harmonics thereof, is transferred as noise to the aural signal. Because the pilot frequency for stereo sound in the U.S. is set at 15.734 KHz, if the noise component added to the aural signal is sufficiently large, the receiver will erroneously perceive this noise as an indication that a stereo signal is being received. As a result, the Federal Communications Commission (F.C.C.) requires that the noise component at 15.734 KHz be 46 db below the reference level for the stereo pilot signal, i.e., the pilot frequency is protected.
Typically, such unwanted phase and amplitude modulation of the aural carrier is prevented, at the transmitter, by using separate picture modulation and sound transmitters, thereby avoiding the coprocessing of the visual and aural signals of the television signal. (See paper entitled "Intercarrier Buzz Phenomena Analysis and Cures" by Fockens et. al., IEEE Trans. on Consumer Electronics CE-27 (3) 381-394 (August 1981).) This separate transmitter arrangement, however, does not prevent unwanted aural carrier distortion and noise at the receiver. Patents directed to addressing correction of the aural carrier distortion at the television receiver include: U.S. Pat. Nos. 3,135,827, 4,237,485, 4,518,997, 4,602,288, and U.S. Pat. No. 4,716,464, as Well as JP 5498118, JP 55-53977, JP 55-99887, JP 59-51676 and JP 59-51677.
Common amplification television transmitter arrangements are known such as disclosed by DE 3306-562-A wherein the audio carrier is mixed with the amplitude modulated picture carrier so that both carriers then have the same phasing errors and these cancel in the intercarrier. However, this arrangement is directed to suppressing phasing errors in the picture carrier rather than addressing the problem of unwanted phase and amplitude modulation or noise in the aural carrier.
Moreover, aural carrier correction circuits currently in use do not independently correct the aural components; but rather, the entire commonly amplified signal is corrected without regard to phase differences that are present between the visual and aural carriers. These phase differences prohibit one correction signal from exactly canceling the unwanted products on both carriers simultaneously.